This disclosure relates to measuring material properties, and, more particularly to, systems and methods for estimating cement acoustic wave speeds.
A wellbore drilled into a geological formation may be targeted to produce oil and/or gas from certain zones of the geological formation. To prevent zones from interacting with one another via the wellbore and to prevent fluids from undesired zones entering the wellbore, a casing may be inserted into the wellbore and sections of the well may be cemented by injecting the annulus formed between the cylindrical casing and the geological formation with cement. The cement ensures the stability of the wellbore, prevents fluid migration between zones of the geological formation, and minimizes the rate of fluid-induced casing corrosion. Injection of the cement involves a number of factors, including the design and the pumping of the cement slurry, contamination of the cement-slurry by the drilling mud, a bad casing centralization, and so on.
Acoustic evaluation of the cement may determine whether cement has been placed in the annulus between the casing and the formation, and whether the cement provides zonal isolation between the formation traversed by the drilled well. A variety of acoustic tools may be used to evaluate the cement after installation. These acoustic tools may include ultrasonic tools operated in fluid-filled casing. For example, an ultrasonic tool, such as Schlumberger Wireline's Isolation Scanner™ tool, may be lowered through the wellbore and rotated to provide vertical and azimuthal imaging of the cement. The cement may be evaluated using acoustic impedance measurements and, for some tools, the casing flexural mode attenuation.
The decay of the flexural mode amplitude is one type of parameter used in the commercial processing of the Isolation Scanner data to determine the cement properties. This decay is estimated from the ratio of the peak of the envelope of the flexural mode amplitude at the two receivers. However, by itself, this energy envelope decay rate leads to an ambiguous answer as the same value can correspond to a cement with a low acoustic impedance as well as one with higher acoustic impedance (ie, the inversion result is double valued). To remove the ambiguity, the decay may be combined with the estimated cement acoustic impedance resulting from processing of the pulse-echo measurement. However, the latter estimation is not always robust due to the strong sensitivity of the inversion method to environmental effects such as mud acoustic impedance as well as to modeling results-based corrections stemming from the inversion reliance on a one-dimensional model for the measurement.